1. Field of the Invention
The field of the invention is that of data compression, notably data for assistance in the reconstruction of an electronic image or picture from a subsampled signal.
The method according to the invention has been particularly designed with a view to the compression of data pertaining to high-definition television pictures, in order to transmit these pictures in channels with reduced pass-bands, for example, according to the D2MAC Packets standard.
A television picture is defined by a number of parameters which represent the spatial and temporal analysis of this picture (in particular, the number of pictures per second, the number of lines per picture and the number of points per line).
The pass-band needed to transmit a high-definition television picture may attain values of the order of 100 MHz.
Thus, to transmit these pictures on channels with reduced pass-bands, (for example, satellite channels having a pass-band of the order of 12 MHz), data compression techniques have to be applied to these pictures in order to obtain a reduction in the transmission throughput rate.
2. Description of the Related Art
There are several known types of picture sub-sampling techniques which enable compression of the transmitted data. For example, there is a known "method for the analysis of structures for spatio-temporal sub-sampling of a HDTV signal with a view to its transmission in an MAC channel" as described in the proceeding of the HDTV Colloquium 1987, Ottawa, 4th to 8th Oct., 1987, Volume 1, pp. 6.2.1. and ff (P. Bernard, M. Veillard, CCETT). In this prior art method of analysis, each picture of the sequence is divided into zones and each zone systematically undergoes three parallel processing modes in three distinct linear filters. Each filter provides for a different sub-sampling filtering operation. These different filtering operations correspond to a preferential filtering operation for still pictures, for moderately mobile pictures and for highly mobile pictures respectively. The outputs of the filters are then compared to the original source, and the best filtering is chosen to determine which of the compressed signals is effectively transmitted.
The throughput rate reduction techniques that are currently being developed are generally aimed at compressing the HD picture so as to obtain a transmission throughput rate that corresponds to a standard picture. In the case of so-called "compatible" sub-sampling, the sub-sampled picture signal transmitted may then meet with two distinct fates:
either a display of the sub-sampled signal on a standard television receiver; PA1 or a reconstruction of a high definition picture from, firstly, the sub-sampled signal and, secondly, additional assistance data. PA1 within the picture of picture image blocks, "neutral" processing mode picture elements are determined, namely picture elements capable of accepting, without discrimination, at least two distinct sub-sampling processing modes; PA1 it is chosen to associate, with each of said neutral processing mode picture elements, those of said neutral processing modes that enable an optimal reduction in the throughput rate of the assistance data.